Calls made by telephones and video telephones are controlled by a call-control system or telecommunication device control system that is usually referred to as a PBX. PBX stands for private branch exchange. While the control systems used by modern VOIP (voice over IP) telephone systems do not exchange anything the term PBX is still commonly used to describe the controllers that have the same core functionality of PBXs used in public switched telephone networks where the term originated.
Modern VOIP telephones need to know the PBX that they are to use to control calls to and from them. There are several known arrangements for providing configuration information to such known IP telephony devices. That is to say, to provide the address or an identifier of the call-control system or telecommunication device control system (PBX) for the IP telephony network. Provisioning of IP telephones falls broadly into three categories, which are described below with reference to FIGS. 1 to 3.
FIG. 1 illustrates a known IP telephone 11 on a telecommunication network which is manually configured with the IP address of the PBX 12 in order to establish the call-control connection 13 across the Internet 10. In the simple arrangement of FIG. 1 the IP telephone is manually told the IP address of the PBX, for example, by a user making an appropriate input using the telephone keypad or the telephone's web interface.
This mechanism allows no flexibility in changing that IP address at a later time without manually reprovisioning all of an organisation's IP telephones, it allows very little flexibility about the location of the PBX relative to the telephone and, furthermore, before first using the telephone a user must make this manual entry, which is often overly complex for a typical end-user to perform.
FIG. 2 illustrates a known IP telephone 11 which is on a network where a dynamic host configuration protocol (DHCP) server 21 provides the IP address of the PBX 12 to the IP telephone at boot time or on start-up via the Internet 10. In this arrangement, the DHCP server informs all the IP telephones on a particular local-area network (LAN) of the IP address of the PBX. To emphasise, the DHCP server is located on the same LAN as the IP telephone being configured. This has some advantages of the arrangement of FIG. 1 in that it is easy to deploy plural IP telephones on a single site, but does not work well for multiple sites or for individual IP telephones, particularly where the IP telephone is being connected to a network not managed by the owner of the IP telephone. It allows some flexibility about the geographical location of the PBX relative to the telephone through techniques such as geographically-aware DNS servers, but it is still complex and cumbersome for a typical end-user to configure.
FIG. 3 illustrates an IP telephone 11 on a telecommunication network which is manually configured with the domain name system (DNS) name of the PBX 12 which it resolves into an IP address using DNS server 31. The arrangement of FIG. 3 uses a DNS server to provide a single level of indirection (means to look up an identifier or pointer to get to the desired destination). The IP telephone is provided with the DNS name of the PBX and the IP telephone then locates a DNS server from its network configuration and queries the DNS server for the address of the PBX. This is achieved by a DHCP server or another device on the local network (not shown in FIG. 3) providing the necessary configuration information to the IP telephone as it boots to allow it to discover the IP address of the PBX. Geolocation DNS services may be used to provide a different PBX address based on geographical knowledge about the IP address of the telephone. This mechanism works for simple systems where there is only one PBX or a group of PBXs all belonging to the same organisation and which can all accept connections from a telephone belonging to that organisation. In other words, it works well for networks that are administered by the same organisation that own the PBX, it provides for operation of an IP telephone without pre-configuration, and is easy for an end-user to deploy, but it is not possible to use on an arbitrary network such as a public or home Internet connection which is typically where ease of deployment is particularly advantageous.